RUF CTR2 was my choice for that race..
as far as I know though, the only thing you're allowed to change on your car are the tires.. and since your computer opponents pitt about every 2/3 laps, just put a set of tires on there that you're comfortable with... I used super slicks, but it took so damned long for them to warm up, it was a pain in the arse driving those first few laps..